Most real protos that are one of a kinds are that way because they were duds. Say a cutlery has a slew of leftover blades from different models that they want to incorporate into a new knife to get rid of excess inventory. They put them together into some new handles, and realize that it looks terrible. Nothing fits or matches, so they toss the idea. Another one might have some new handles that they were trying out, but realized that they looked terrible, or warped, or cracked, and tossed the idea. On the other hand, the really collectable (and rare) protos are the ones sent in to the cutleries by custom makers who are involved in partnerships. One example I've seen is an D'Alton Holder proto of the millenium series knife. It was much bigger, with more defined blades and more handle detail. A real custom. Once Schrade got the proto they whittled it down some to make it more affordable and cost efficient, but that proto was a beauty. I've also seen the one he sent in to Canal Street for their new bowie, although this time the production model is very close to his design.
Eric